Closing error; plotting methods in compass traverse

Closing error: when a closed traverse is plotted, the last point often falls at A′ instead of at the starting point A; the distance A′A is the closing error. The ratio (closing error / perimeter) shows the accuracy of the survey.

Methods of plotting a compass traverse

1. Included (interior) angle method

• Calculate the interior angles from the FB and BB, plot the first line to scale, and at each station set out the angle with a protractor to plot the next line. Angular errors accumulate from station to station, but the sum of the angles can be checked first. Suitable for a closed traverse.

2. Bearing (protractor) method

• Draw a meridian line through each station and plot each line directly at its observed bearing with a protractor (circular or paper protractor), laying off the lengths to scale. It is simple and quick, and errors carry from one line to the next less than in the angle method, but the accuracy is limited by the protractor.

3. Tangent method

• A convenient length is laid off along the meridian (or along the previous line) from the station, and at its end a perpendicular offset equal to that length × tan(bearing or angle) is set off to scale. The line joining the station to this point gives the direction of the survey line. It is more accurate than using a protractor.

4. Co-ordinate (latitude and departure) method

• Compute the latitude $L\cos\theta$ and departure $L\sin\theta$ of every line, find the co-ordinates of each station, and plot the stations on a grid. It is the most accurate method, and the closing error can be adjusted by Bowditch's rule before plotting.

After plotting, the closing error A′A is adjusted by Bowditch's rule.